It is a common practice to characterize materials by sending an energy wave, comprising some form of energy, such as acoustic energy, through the material to be characterized. This technique is used in geology, for characterizing a volume of earth in search of such materials as oil and water deposits. The techique is also used in the technology relating to non-destructive evaluation of structural materials, to detect imperfections, such as voids, cracks, or inclusions, in materials such as steel, aluminum and concrete.
This technique employs an energy source for generating an energy pulse signal which travels through the material to be characterized. A receiver is employed for receiving and interpreting the pulse or signal. The distance between the source and the receiver is known, as is the energy of the transmitted pulse. By measuring the time it takes for the signal to travel from the source to the receiver, the velocity of propagation can be computed and compared with tables of known characteristics of materials. In this way, it is possible to characterize the type of material. By making a series of measurements along different paths through the material, and by detecting and characterizing echos, it is possible to detect and characterize irregularities in the material.
In geological exploration, the energy source of the receiver may be placed at different locations on the surface of the earth, or in first and second bore holes which are spaced apart by a known distance. The source or generator may produce signals in the form of a series of discrete pulses which travel the known distance from the generator to the receiver. Typically, the generator may consist of such devices as dynamite or other explosive charges, or a source of electric sparks. Such generators may produce pulses comprising compression or pressure waves, rarification waves, and shear waves having both horizontal and vertical components, as desired. Shear waves are often used as the energy pulses because sheer waves travel at approximately one half to one third the rate of speed of compression waves, thus allowing more time for the receiver to receive and analyze the pulses.
The existing techniques suffer from problems and disadvantages, including operational difficulties in setting up and using the existing techniques, difficulties in making accurate measurements of the propagation times of the pulses, and difficulties in interpretation of the test data, so that interpretation by highly skilled personnel is required. The comparison of the test data with the known material characteristics is also difficult.